Oxydehydrogenation process for preparing methacrylic acid and its lower alkyl esters

ABSTRACT

Isobutyric acid or a lower alkyl ester thereof is oxidatively dehydrogenated in the vapor phase producing the corresponding α,β-olefinically unsaturated derivative by contact with a heterogeneous catalyst in the presence of molecular oxygen. The catalyst is composed of the calcined phosphates of iron and copper.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the conversion of isobutyric acid or itsequivalents and lower alkyl esters thereof correspondingly tomethacrylic acid or its equivalents and lower alkyl esters thereof.

2. Description of the Prior Art

There exists considerable prior art relating to the oxydehydrogenationof the lower saturated aliphatic monocarboxylic acids to produce thecorresponding α,β-olefinically unsaturated acids. Early work in thisarea involved thermal, vapor phase oxydehydrogenation of the saturatedaliphatic carboxylic acid in the presence of oxygen and iodine. Thisapproach has not been particularly successful from a commercialstandpoint. This is understandably so inasmuch as iodine is costly,exhibits extreme corrosivity properties and poses considerable problemsin realizing complete recovery of the comparatively large amountsthereof required in the process. The heterogeneous catalytic method foroxydehydrogenation according to the prior art appears to be the moreattractive route to the commercial production of olefinicallyunsaturated monocarboxylic acids. The prior art heterogeneousoxydehydrogenation catalysts useful for this purpose include someheteropoly acids, such as phosphomolybdic acid, optionally whichtungsten and/or vanadium. Another type of catalyst included in the priorart is iron phosphate.

Iron phosphate subjected to calcination exists in several crystallinephases or species. It is not known at this time which species is or arecatalytically active. There is evidence that the presence of certainextrinsic metal components in the catalyst preparation serves tofacilitate the formation of the active catalyst. For instance, U.S. Pat.No. 3,948,959 discloses that an alkali or alkaline earth metal can bethe extrinsic metal component for this purpose.

SUMMARY OF THE INVENTION

In accordance with this invention, a catalytic process is provided forthe oxidative dehydrogenation of a saturated aliphatic monocarboxylicacid or lower alkyl ester thereof, such as isobutyric acid or methylisobutyrate, to the corresponding α, β-olefinically unsaturatedderivative thereof, such as methacrylic acid or methyl methacrylate. Theprocess of this invention comprises contacting a heterogeneous catalystat a temperature in the range of from 300-500° C. with a mixture of thesaturated aliphatic monocarboxylic acid and molecular oxygen, saidcatalyst being a calcined copper-iron phosphate. The catalyst useful inthe process of this invention can be further defined by the gram-atomempirical formula Cu_(a) Fe_(b) P_(c) O_(x) wherein a is 0.01-1.0, b is0.1-1.0, c is 0.01-2.0 and x represents the number of oxygens requiredto satisfy the uncombined positive valences of the other elements shownin the formula.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There are a number of techniques which can be used for preparing thecatalyst useful in the process of this invention. Of these, the morefacile methods involve preparing the integral catalyst composition priorto calcination. This can be readily accomplished by the so-called slurrymethod or the precipitation method. In the latter method an aqueoussolution of salts of the metals and phosphoric acid is first preparedand is then neutralized with an appropriate base so as to precipitatethe mixed metal phosphates. The precipitate is washed and dried prior tocalcination. In the alternative, one can add ammonium phosphate to asolution of the metal salts causing direct precipitation of the metalphosphates. As indicated, any water-soluble salt of iron and copper canbe used in the preparation of the solution. The nitrate salts aresuitable water-soluble salts for this purpose and are preferred becauseof their ready availability and desirable solubility characteristics.

The so-called slurry method is the preferred catalyst preparation methodbecause of its convenience. In accordance with this procedure, theaqueous solution of the iron and copper salts together with thephosphoric acid is obtained as previously noted. The solution is thenheated with stirring to remove water and this is continued until themass is so thick it cannot be stirred. The resulting residue is thenbroken up and heated to a moderately elevated temperature in the orderof about 120° C. until the mass is completely dried. The resulting solidis sized and calcined. Suitable calcination temperatures range from400-500° C. Applicable periods of calcination range from 2-30 hours.

The use of a support or carrier for the catalyst is included in thisinvention. The support can be included in either of the catalystpreparation methods mentioned above. For instance, in the slurry methodcolloidal silica or any other form of silica as well as other supportmaterial such as alumina, pumice, quartz, zirconia, titanium dioxide,carbon, silicon carbide, etc. can be included prior to the water removalstep. Similarly, the precipitation of the metal phosphates can beaccomplished in the presence of suspended particles of the supportmaterial in the alternate method described above.

The process of this invention can be carried out using the catalyst inthe form of a fluidized bed reactor, a stirred tank reactor or in afixed bed or packed bed reactor or any combination of these types ofreactors. Because of the convenience associated with the use of a fixedbed reactor in a small scale operation, such a reactor will beexemplified herein. In the preferred mode of operation the feed to thereactor comprises a preheated gaseous mixture of the saturated aliphaticmonocarboxylic acid, molecular oxygen, steam and inert diluent gas. Apreheat temperature in the range of about 300° to 350° C. is customarilyused. The oxydehydrogenation reaction can be carried out in the range offrom 300° to 500° C. More generally a temperature of from 375° to 450°C. provides for optimum processing.

The mole ratio of molecular oxygen to carboxylic acid is from 0.5 to 1.5and more preferably from 0.7 to 0.75 in the case where the carboxylicacid is isobutyric acid, per se. Although steam is not necessary for thereaction, its presence is desirable in the feed because it is believedto act beneficially as a heat sink and in minimizing combustion of thecarboxylic acid to undesirable waste products. The mole ratio of waterto the carboxylic acid in the feed should be from about 8 to 20. Theoptimum ratio is from 12 to 15. Acetone may be included in the feed inthe conversion of isobutyric acid to methacrylic acid.

Another important parameter is the concentration of the organic reactantin the feed. The organic reactant carboxylic acid or ester should bepresent in the feed in from 0.1 to 20 mole percent. From the standpointof achieving a reasonable throughput combined with an acceptable yield,the concentration of the reactant in the feed is from about 3-6 molepercent. Concentration of reactant in the feed is controlled to a largedegree by the amount of inert gas present. The preferred inert gas ordiluent is nitrogen although other inert gases such as carbon dioxide,helium, argon, and the like are suitable. Air is a very convenientsource of oxygen plus inert diluent.

Another important parameter is contact time in the process of thisinvention. Contact or reaction time is defined for the purpose of thisinvention as the catalyst volume divided by the volume of gas feed persecond at the reaction temperature. The catalyst volume is the bulkvolume occupied by the catalyst in the reactor. The term catalyst inthis sense not only includes the copper-iron phosphate itself but alsothe support if present. Accordingly, reaction times can range from 0.05to 3.0 seconds and more generally in the order of from 0.1 to 1.0second. The reaction is preferably carried out at or near atmosphericpressure although the use of higher pressures up to about 10 atmospheresis contemplated.

The process of this invention is further illustrated in the followingspecific examples.

EXAMPLE I

The slurry method was used to prepare a copper-iron-phosphate catalyston a silica carrier. In 400 ml. of water were dissolved 120.8 g. ofcopper nitrate and 202 g. of iron nitrate. To this solution were added143 g. of 85% concentrated phosphoric acid. To this mixture were thenadded 30 ml. of an aqueous (40% by weight solids) silica gel. Water wasremoved from the resulting slurry with stirring, the solid was furtherdried at 120° C. for 6 hours and the treated solid was calcined at 450°C. for 6 hours (in air). The final catalyst was found to be blue and hadthe empirical formula Cu₀.5 Fe₀.5 P₁.24 O₄ /SiO₂ 3.5%.

EXAMPLE II

This example illustrates the use of the catalyst described in Example Iin the oxydehydrogenation of isobutyric acid to produce methacrylicacid. The procedure consisted of feeding a preheated mixture ofisobutyric acid, oxygen, nitrogen and steam through a stainless steeltube of 1/2" O.D. (3/8" I.D.) and approximately 18" in length containingthe catalyst as a 12 cc. packed bed maintained at the reactiontemperature.

The preheater consisted of a length of stainless steel tubing similar tothe reactor but packed with glass beads. Any carbon dioxide formed inthe course of the reaction was absorbed in an Ascarite tube protected bya calcium sulfate absorber for any uncondensed water. The condensedorganic product was separated from the water, collected and analyzed bythe internal standard method of gas chromatography.

Selectively to methacrylic acid represents the mole ratio of methacrylicacid found in the reaction effluent to that of the isobutyric acidconsumed in the reaction.

The feed to the reactor consisted of isobutyric acid:oxygen:water:nitrogen in the corresponding mole ratio of5.4:3.9:64.8:25.9. The reaction temperature was 430° C. and the contacttime was 0.56 second. In this experiment it was found that 81.7% of theisobutyric acid was converted to other products and the selectivity tothe product methacrylic acid was 67.6%.

EXAMPLE III

The procedure of Example II was followed except that the mole ratio ofisobutyric acid to water in the feed was 6, the reaction temperature was428° C. and the contact time was 0.6 second. In this experiment it wasfound that 75.1% of the isobutyric acid was consumed with a 63.4%selectivity to methacrylic acid.

EXAMPLE IV

The procedure of EXAMPLE III was followed using a contact time of 0.5second and the conditions shown in Table I. The results obtained areshown in Table I.

                  TABLE I                                                         ______________________________________                                                                         % Selectivity                                Isobutyric Acid/                                                                         Tem-     % Conversion of                                                                            to Methacrylic                               Water(ML/Hour)                                                                           perature Isobutyric Acid                                                                            Acid                                         ______________________________________                                        8.4/20     435° C.                                                                         81.7         67.6                                           8.8/19.1 450° C.                                                                         71.3         66.8                                         8.8/20     435° C.                                                                         69.0         68.3                                         ______________________________________                                    

EXAMPLE V

Another catalyst was prepared according to the procedure of Example I.In 400 ml. of water were dissolved 60.4 g. of copper nitrate and 303.4g. of ferric nitrate. To this solution were added 143 g. of 85%concentrated phosphoric acid and 30 ml. of a 40% solids aqueous silicasol. Water was removed from the resulting slurry with stirring, thesolid was further dried at 120° C. for 6 hours and the thus treatedsolid was calcined at 450° C. for 6 hours in air. The final catalyst wasfound to be blue and to have the empirical formula Fe₀.75 Cu₀.25 P₁.24O₄ /SiO₂ 3.5%

EXAMPLE VI

The procedure of Example II was followed with the feed ratios andreaction temperatures shown in Table II at a contact time of 0.56second. The results obtained are given in Table II.

                  TABLE II                                                        ______________________________________                                                   Tem-                                                               Isobutyric Acid/                                                                         pera-   % Conversion of                                                                            % Selectivity to                              H.sub.2 O (Ml./Hour)                                                                     ture    Isobutyric Acid                                                                            Methacrylic Acid                              ______________________________________                                        8.4/20     410° C.                                                                        65.2         65.7                                          8.4/20     400° C.                                                                        53.0         71.5                                          5.5/15     400° C.                                                                        49.0         85.2                                          ______________________________________                                    

I claim:
 1. In a process for the catalytic conversion of isobutyric acidor a lower alkyl ester thereof to the corresponding α,β-olefinicallyunsaturated derivative by oxydehydrogenation wherein a catalyst iscontacted with a gaseous feed stream containing said acid or ester andmolecular oxygen at a temperature between about 300° and 500° C.; theimprovement comprising using as catalyst a material having the gram-atomempirical formula Cu_(a) Fe_(b) P_(c) O_(x) wherein a is 0.01-1.0, b is0.01-1.0, c is 0.01-2.0, and x is determined by satisfying the sum ofthe unshared positive valences of the other elements shown in theformula.
 2. The process of claim 1 wherein isobutyric acid is convertedto methacrylic acid.
 3. The process of claim 2 wherein the catalyst hasthe gram-atom formula Cu₀.25 Fe₀.75 P₁.24 O₄.